1. Field of the Invention
This invention relates in general to communication networks and in particular to a method, apparatus and system for rapidly acquiring remote communication nodes in a communication network (e.g., satellite communication network).
2. Discussion of the Background
In communication systems having a communication segment (e.g., a communication link) between a hub and one or more remote communication terminals (“remotes” or “remote nodes”) where the communication segment has a relatively high latency time (e.g., a satellite communication system), a disadvantageously long time may be required to acquire the remote nodes and bring them into the communication network. Remote nodes are considered to be out of the network when they are powered down, or recently powered up, or for some other reason have lost communication with the network. Remote nodes are considered to be in a network when they are configured to transmit a signal using a communication frequency and a symbol offset timing that allows the transmission to be recognized and understood by the hub. Moreover, a remote node is in the network when it is configured to send and receive signals in a fashion that is consistent with the network's protocol and timing. The process of converting out of network remotes to in network remotes is known as remote acquisition. The relatively high latency of the communication segment has conventionally resulted in long delays while remote nodes are acquired.
For example, a conventional communication system may include a satellite network having a star or star/mesh topology where the network control entity is located at a central hub earth station and remote earth stations are connected to the hub via satellite link. In such a satellite network, the hub transmits a continuous downstream carrier such that all remotes in the satellite's footprint can receive its transmission. The remote nodes transmit bursts of data to the hub using a Time Division Multiple Access (TDMA) communication scheme. When a remote is first powered on, performs a reset, comes out of a rain fade condition, or otherwise has lost contact with the hub, it must be reacquired. The process of acquisition involves the remote and hub performing station-keeping operations with the goal of preparing the remote to transmit TDMA bursts such that they can be received reliably without corrupting other users of the TDMA channel or neighboring users on other portions of the satellite or on neighboring satellites. Various frequency modifying conditions may cause an uplink transmission from a remote to a hub to arrive at the hub having a different frequency than expected. Frequency modifying conditions may include, for example, differences in cabling, radio frequency (RF) equipment, temperatures, or even satellite motion that causes the frequency to change or to be different than the frequency expected at the hub.
Background acquisition techniques vary from simple to complex, depending on the sophistication of the hub receive circuitry and control mechanism. A very simple and commonly used technique is ALOHA access where the remote terminals burst data randomly to the hub receiver and rely on there being a low probability that no other user will access the channel at that instant. Collisions between data bursts result in data corruption requiring the colliding users to retransmit lost data. Even in an ALOHA system, however, provisions must be made to ensure that the remote's local frequency reference is stable and that its power level is high enough to close the link, but not too high to introduce inter-modulation interference on the satellite transponder.
Different background techniques may be used for TDMA systems where remotes are assigned specific data communication slots in a predefined time frame in which to transmit. One background technique is for a remote to transmit a very short burst (much shorter than its time slot) into its assigned data communication slot. The remote adjusts parameters as directed until the hub determines that the remote is ready to transmit full length traffic bursts in its assigned data communication slot without interfering with other users.
Another background technique involves designating a portion of the TDMA frame as an acquisition slot with additional guard symbols (i.e., the acquisition slot is longer than the acquisition response burst intended to be received in the acquisition slot) to allow remotes to burst an acquisition response with full sized bursts until the station keeping parameters are adjusted.
ALOHA based acquisition inherently breaks down due to an increasing frequency of collisions when the channel use exceeds 18% capacity. Networks configured to operate at larger capacity use predefined slots and a slot assignment methodology and a corresponding acquisition procedure. Background acquisition procedures require multiple “handshakes” back-and-forth between hub and remote in order to bring all station keeping parameters (e.g., remote transmit frequency, symbol offset and power) under control. These handshakes involve waiting for commands and responses to propagate across the long latency satellite channel where the round trip time is as much as half a second. Thus, in the background systems, total acquisition time may be directly proportional to the latency of the communication network, and therefore background communication systems may have a significant scalability problem. Background acquisition algorithms may require several round trip times to complete a single acquisition (some implementations require 20 or more). For example, in a background satellite network requiring three round trips to acquire a single remote, the total time to acquire a remote would be approximately 1.5 seconds. Thus, a network of 1000 remote sites would require 1500 seconds (25 minutes) to fully recover from a network wide reset (less efficient implementations take many hours).
Alternative background systems have addressed this issue by adding complexity and cost to the hub controller such that it can receive many different overlapping messages simultaneously, however the total acquisition time may still be proportional to latency.
FIG. 11 shows an example of a communication network including hub 1100 configured to communicate using communication segment 1102 with a remotely located remote 1104. The communication segment 1102 has a relatively high latency time, for example, a latency of approximately 250 ms in one direction, as in a satellite communication network. Thus, in this example it may take approximately 250 ms for a signal sent by hub 1100 to be received by remote 1104. FIG. 12 shows a further example of a communication network including hub 1300 that is configured to communication via communication segment 1302 with three remote nodes, remote R0 1202, remote R1 1204 and remote R2 1206.
FIG. 13 shows a conventional method for acquiring a single remote in a communication network similar to that of FIG. 11 or 12. In this example to acquire the remote, a hub sends an acquisition command (S1300). The acquisition command is received by the remote and the remote sends an acquisition response (S1314). However that response is not received by the hub because the remote did not send the response using the appropriate communication parameters (e.g., frequency, symbol offset, or power levels) needed for the hub to receive the transmission. Accordingly, when the hub listens for the acquisition response (S1302), the hub does not receive the response. The hub may try again to send a different acquisition command (S1404) which may be received by the remote (S1316), but again the hub does not receive the acquisition response when it listens for the acquisition response (S1306) because the set of parameter values used by a remote for this transmission is still not suitable for allowing the hub to recognize its transmission. Finally, the hub sends acquisition command (S1308) which is received by the remote (S1318) and the acquisition response (which this time uses the correct parameter values) sent by the remote is received by the hub when it listens for the acquisition response (S1310). In this example, the amount of time required to acquire the first remote is shown as acquisition time 1320, which is composed of plural round trip communications that take round trip time 1322 to communicate from the hub to the remote and back to the hub again. In this conventional method, acquisition of the first remote completes prior to the beginning of acquisition of the next remote, for example the first remote must be acquired before sending an acquisition command (S1312) to start the acquisition process for a next remote.
Thus, in a conventional communication system with plural remotes, for example, the communication system shown in FIG. 12, the acquisition time for all of the remotes in the system (“system acquisition time”) may be as much as the acquisition time for a single remote (e.g., acquisition time 1320) times the total number of remotes in the system.
Node acquisition may be a frequent occurrence in a background communication system. For example, in a satellite communication system a remote may become out of network due to weather or equipment conditions and plural remotes may become out of network when a maintenance condition or equipment failure causes the hub to be restarted or when plural remotes are remotely upgraded, for example. Further, in the background approach a great deal of time may be spent bringing remotes into the network (i.e., acquiring remotes) and during that acquisition time normal communications may be interrupted.